Dynamics: Damping

Damping ratio obtained from forced vibration tests

The forced vibration tests of the framed steel building for obtaining its resonance frequency are described in Section 16.4.3. The frequency spectrum obtained from the experiment and curve fitting are shown in Fig. 16-17.

The natural frequency and damping ratio obtained from the forced vibration tests were 0.617 Hz and 2.25% respectively. It can be noted that the measured values have a characteristic negative skew compared to the best-fit curve. This is typical of this type of measurement and shows one aspect of nonlinear behaviour as observed in free vibration tests of buildings and other structures.

It can be observed from Table 9-1 that the measurements from the forced vibration tests agree favourably with those obtained from the free vibration tests for relative amplitudes between 0.366 and 1.000. In general, forced vibration tests normally provide larger forces than free vibration tests. Therefore forced vibration tests frequently produce smaller values for natural frequencies and larger values for damping ratios than those obtained from free vibration tests.